Note: Descriptions are shown in the official language in which they were submitted.
-- 1 --
"AERATION OF LIQUIDS"
TECHNICAL FIELD
This invention relates to aeration of liquids
especially to effect absorption of a gas into the liquid,
stripping of a dissolved gas from the liquid, the
separation by flotation of droplets or particles suspended
in the liquids, or the formation of a cloud of bubbles to
be transported into a reservoir of liquid to carry out any
of the above functions.
BACKGROUND ART
In each of the operations referred to above it is
advantageous to first disperse the gas into fine bubbles
in order to create a large interfacial area for transfer
of matter between phases, or for collection of the
particles by flotation. Having created a dense dispersion
of fine bubbles in the liquid, it is further advantageous
to inject the gas-liquid mixture into a reservoir in which
further transport operations such as absorption, stripping
or flotation, can take place. In the past this has been
achieved by many different types of diffusers, nozzles, or
jets or various forms of mechanical impellers and other
aerators, but they all have significant limitations in the
size of the bubbles produced, the flow rate of bubbles,
and the dispersion of the bubbles into the body of liquid
to be aerated.
It is the purpose of the method and apparatus for
the aeration of liquids according to this invention to
provide a de~ice for the dispersion of a flow of gas into
a mi~ture of fine bubbles in a liquid, and then to provide
a means for efficient usage of the bubbles in a large
reservoir of the liquid.
DISCLOSURE OF INVENTION
Accordingly in one aspect the invention consists in
a method of aerating liquids comprising the steps of
passing a liquid in a downwardly moving jet into the upper
part of a downwardly extending conduit, the lower end of
which is submerged in a body of liquid, forming a foam of
bubbles within the conduit and causing the foam to move
~"~ "~ i?
a~~
? ~
-- 2 --
downwardly and issue from the lower end of the conduit
into the body of liquid.
Preferably gas is provided to the upper part of the
conduit and entrained into the jet forming the foam of
S bubbles.
Preferably the upper part oE the conduit is
enclosed and the gas is provided at atmospheric pressure
through a flow rate controlling constriction.
Alternatively the upper part of the conduit is
enclosed and the gas is provided under pressure via a pump
or blower.
In one form of the invention the liquid passing
into the upper part of the conduit incorporates a gas in
supersaturated solution within the liguid.
The gas may be any convenient or desired gas but in
many applications comprises air.
In a further aspect the invention consists in
apparatus for aerating liquids comprising a substantially
vertically extending conduit having an open lower end,
liquid supply means arranged to supply liquid under
pressure to a downwardly facing nozzle loc~ted and
arranged within the upper part of the conduit so as to
form a downwardly issuing jet of liquid within the
conduit, and support means arranged to support the conduit
with the iower end immersed in a body of liquid.
Preferably the conduit is provided with a diffuser
extending outwardly from the exterior of the conduit and
positioned to be immersed in the body of liquid, the
diffuser being arranged to disperse bubbles issuing from
the lower end of the conduit outwardly away from the
conduit as the bubbles rise in the body of liquid.
Preferably the diffuser comprises a plate
surrounding the conduit and extending outwardly and
upwardly from a predetermined position on the exterior of
the conduit.
Preferably the plate is provided with a plurality
of holes thsrethrough sized to allow a predetermined flow
rate of bubbles through each hole.
-- 3 --
Preferably a gap is provided between the inner
periphery of the plate and the e~terior of the conduit and
a diverting ring is positioned on the e~terior of the
conduit below the gap, arranged to divert upwardly moving
bubbles outwardly away from the conduit and away from the
gap between the conduit and the plate.
In one form of the invention the interior of the
conduit is provided with a draft tube having its axis
parallel to the axis of the conduit and being configured
to direct the flow of gas within the conduit and/or
constrain the jet within the conduit.
Preferably the conduit is substantially circular in
cross section or is of other cross-section having minor
and major lateral axes of the same order and having an
effective diameter equal to the diameter of a circle of
equivalent area, and wherein the diameter of the conduit
is in the range 2 to 20 times the diameter of the nozzle.
More preferably the diameter of the conduit is
within the range 3 to 12 times the diameter of the nozzle.
The liquid to be aerated may be either the liquid
which is passed into the upper end of the conduit through
the nozzle in a downwardly moving jet or alternatively may
be the liquid comprising the body of liquid into which the
lower end of the conduit is immersed. In many
applications the liquid issuing into the upper part of the
conduit will be the same as the liquid in the body of
liquid.
Examples of the use to which the invention may be
put include the following:
(a) Aeration of a pond of wastewater or effluent,
in which it is desired to create a dispersion of fine
bubbles in the wastewater, so as to provide oxygen for the
growth of microorganisms used to remove the noxious
components and hence reduce the biological oxygen demand
prior to discharging into a sewer or river.
~ b) Treatment of a water or effluent stream
containing finely-dispersed oil droplets, so that the oil
droplets attach themselves to the bubbles, which rise to
U ~ - 4 -
the surface of the liquid in a pond or containing vessel,
removing the oil droplets out of the main body of the
liquid, which is therefore purified.
A fine dispersion of bubbles is created in the
conduit or pipe which is substantially vertical, by the
action of a jet of the liquid impinging into a column of
dense foam in the vertical pipe. The bubbles are created
at least in part by the shearing action between the
high-speed jet of liquid and the relatively quiescent
dense foam.
The velocity of the jet is sufficiently high to
disperse the air entrained into the dense foam into very
small bubbles, of ma~imum size 500 ~m approximately.
Such fine bubbles will provide a medium with a high
interfacial area. Furthermore, in the absence of larger
bubbles, the two-phase mixture thus created is found to
flow as an essentially homogeneous stable mixed fluid,
creating a favourable environment for absorption,
flotation and kindred processes.
The jet of liquid is supplied to the top of the
vertical pipe, so that the two-phase mi~ture created in
the pipe is forced to travel downward, and then to
discharge into a large reservoir. When the mi~ture
dischargPs out of the end of a parallel-walled vertical
pipe, it has been found that the resulting flow is not
conducive to the most effective use of the bubbles. The
finely-dispersed dense foam tends to act as a single
homogeneous phase whose density is very much less than
that of the liquid into which it is discharging, and
accordingly the bubbly foam tends to cling to the outer
wall of the vertical pipe and rise quickly to the surface
of the liquid in the reservoir.
Better use of the bubbles can be made if the
homogenous foam flowing from the bottom of the vertical
pipe, can be made to mi~ with the surrounding liquid, so
that the bubbles become separated from each other and rise
as if they were essentially in an infinite body of
liquid. Being very small, the rise time of the individual
. _
-- 5 -- ..
bubbles is much greater than the rise time of the
homogeneous mixture issuing from the bottom of the
vertical pipe, and accordingly, the time available for
molecules to diffuse in or out of the bubbles is greatly
increased.
It is one of the purposes of this invention, to
provide a means for the efficient dispersion of a gas into
a stream of fine bubbles intimately mixed into a liquid
stream to form a dense foam, through the use of the
shearing action of a high-speed liquid jet confined in an
essentially vertical pipe which discharges into a
reservoir of liquid.
It is a further purpose of this invention, to
provide simple and effective means for the individual
bubbles in the essentially homogenous mixture issuing from
the bottom of the vertical pipe, to be separated from each
other so as to rise slowly to the surface~ giving a
greatly enhanced time of contact between the bubbles and
the liquid in the reservoir, and hence giving maximum
opportunity for transfer of material between the phases,
or capture of fine particles by the bubbles.
BRIEF DESCRIPTION OF DRAWINGS
Notwithstanding any other forms that may fall
within its scope, one preferred form of the invention and
variations thereof will now be described with reference to
the accompanying drawings, in which:
Figure la is a diagrammatic cross-sectional
elevation through a basic form of aeration device
according to the invention;
Figure lb is a view similar to Figure la showing
the behaviour of a bouyant plume of dense foam in the
absence of a diffuser;
Figur~ lc is a sectional plan view of the difuser
incorporated in the apparatus of Figure la;
Figure 2a is a diagrammatic cross-sectional
elevation through the lower part of the conduit forming
part of the apparatus of Figure la showing an alternative
embodiment of the diffuser;
~ U V -- _
-- 6
Figure 2b is a plan view of the diffuser shown in
Figure 2a;
Figure 3a and Figure 3b ~re diagrammatic
cross-sectional elevations of alternative forms of
diffuser;
Figure 4a is a diagrammatic cross-sectional
elevation showing aeration apparatus with multiple nozzles
and internal baffles in the conduit;
Figure 4b is a sectional plan view of the nozzle
and ~affle arrangement shown in Figure 4a;
Figure 5 is a diagrammatic cross-sectional
elevation of a form of the invention incorporating a draft
tube within the conduit;
Figure 6a and Figure 6b are views similar to Figure
5 showing alternative configurations of draft tube; and
Figure 7 is a view similar to Figure 5 showing an
alternative draft tube configuration.
MODES FOR CARRYING OUT THE INVENTION
In the form of the invention shown in Figure la,
liquid enters through an entry pipe (1) and a nozzle
assem~ly (22) which terminates in an orifice (2) which
faces essentially vertically downwards. The nozzle is
mounted in the top of a conduit or pipe (3~ which is
essentially vertical. In operation, the liquid issues
from the orifice (2) in the form of a high-speed jet which
can move downwardly through the pipe (3).
The vertical pipe is mounted by way o support
means (3a) so that its lower end is submerged in a
reservoir of liquid (4). The liquid may or may not be the
same as the liquid entering through the entry pipe (1).
Initially, before commencement of operation, the
liquid levels in the reservoir and inside the vertical
pipe (3) are the same. When the high-speed liquid jet is
first established by the orifice (2), it travels downwards
through the pipe (3) and plunges into the liquid,
entraining gas which is inside the pipe and carrying it
out of the lower extremity (5), to rise in the reservoir
(4) in the form of fine bubbles. The vertical pipe (3)
7 ~ ' t / ~t~r~
~ 'U i'~
-- 7 --
fills rapidly with a dense foam of bubbles dispersed in
the feed liquid, and the pressure in the head space of the
pipe drops below the ambient pressure outside the pipe.
Accordingly, new gas, which is typically but not
necessarily air, is drawn into the pipe through the air
entry (6).
The gas flow is regulated by a conveniently placed
control valve (7) or other suitable means so that the rate
at which air enters through the entry pipe ~6) is less
than the ma~imum amount which can be entrained by the
plunging jet (8). In this way, the vertical pipe (3)
remains filled with a dense foam which provides a
favourable environment for interaction between the gas and
liquid phases.
lS The pipe (3) in which the bubbly mixture is
produced should preferably be substantially vertical, i.e.
within 15 of the vertical. It is however possible for
the system to perform well in some cases when the axis
lies further from the vertical than the limit stated,
depending on the degree of coalescence of the bubbles
which takes place within the pipe as the dense bubbly foam
travels downward toward the pipe e~it (5). If the bubbles
coalesce, they will rise in the form of large slugs of gas
in the uppermost parts of the sloping pipe, to the head
space of the pipe (3), as a form of internal gas recycle.
On reaching the head space in the pipe (3), they will
displace the dense foam, and may cause the collapse of the
bubbly mixture in the pipe. Accordingly, the pipe (3)
will perform best if it is substantiall~ vertical.
Although the invention is described with reference
to a circular pipe (3), it is not restricted to this form,
and indeed the pipe may be replaced by a vertical duct of
any cross-section. Best results will be found however
with a regular polygon, or a section for which the ratio
of the major to minor lateral a~es is close to unity.
The apparatus has been described in terms of the
formation of a dispersion of gas bubbles in a liquid, by
entrainment of the gas into a dense foam mixture by the
action of a high-speed liquid jet. In some applications,
the gas is supplied entirely by the entrainment process.
However, in the flotation of fine particles it is
advantageous to supply gas not only in the form of bubbles
created by the high-speed jet, but also by growth from a
supersaturated feed solution. There is a substantial drop
in pressure as the liquid passes through the injection
nozzle. Thus if the feed liquid is saturated with the
flotation gas prior to passing through the nozzle, it will
become supersaturated downstream of the nozzle and the
dissolved gas will come out of solution in the form of
very fine bubbles. The presence of these bubbles aids the
flotation process because the surface area per unit volume
of such bubbles is very large, and also because the
lS bubbles may preferentially nucleate on the surface of the
fine hydrophobic particles which it is desired to float.
Accordingly, the ability to float the fine particles with
bubbles will ~e enhanced by the use of supersaturated feed
li~uid.
When dissolved gas is used to enhance the flotation
process, there is a synergistic effect between the buhbles
produced by the shearing action of the plunging jet, which
are normally around 500~m in diameter, and the bubbles
produced by growth from the supersaturated solution, which
~5 are often in the range 20 to 50~m in diameter. Because
the latter bubbles are so small, their rise velocity is
also small, and hence the time required for them to rise
to the surface of the liquid after issuing from the bottom
of the circular pipe (3) can be very large. However, when
there is a mixture of bubble sizes, coalescence can occur
between the small and large bubbles to produce bubbles of
even larger diameter, and so the rise time to the surface
can be reduced, and hence the volume of the reservoir into
which the pipe (3) is discharged can also be reduced.
In some applications, the only supply of gas may be
by way of dissolved gas in the liquia passing through the
nozzle assembly (22). The top of the conduit or pipe t3
may be sealed and the inlet (6) omitted.
W~ ~7/lll~o f~ 3 3 ~ .L ~ '- ' ' "' `~';'!'!``'
The gas-liquid mixture which forms in the vertical
pipe (3) has a voidage of up to 60 percent by volume
approximately, and behaves as a homogeneous fluid whose
density is much less than the liquid in the reservoir
t4). Accordingly, in the absence of any special
precautions, it tends to rise as a buoyant plume, hugging
the outer wall of the pipe ~3) and rising rapidly to the
surface ~9), as shown in Figure lb. Consequently, the
bubbles in the plume do not mix well with the liquid in
the reservoir, and the possibility of maximum contact of
the bubbles with this liquid is lost. To prevent the
formation of the plume, it has been found advantageous to
mount a shield or diffuser (10) outside the vertical pipe
(3), which has the effect of breaking up the dense foam
which issues from the pipe exit (5), and allowing the gas
bubbles to rise individually in the liquid in the
reservoir (4) rather than as a plume.
The bubble diffuser can conveniently be made from a
plate in the form of a frustum of a cone, inverted so that
the wide end is uppermost. The cone is perforated with a
multiplicity of holes as shown in Figure lc. In
operation, the buoyant plume issuing from the exit (5) of
the vertical pipe ~3),rises and spreads over the underside
of the diffuser (10), to pass through the individual holes
(11) in the diffuser (10). After passing through the
~- individual holes (11), the bubbles of gas rise
individually through the liquid in the reservoir (4) as
shown in Figure la.
The diameter of the conical diffuser (10) shown in
Figure lc should be in the range 1.5 to 10 times the outer
diameter of the pipe (3), with good practical results
being found when the cone diameter is 2 to 3 times the
pipe diameter. The half-angle of the cone from which the
frustum is formed is conveniently in the range 30 to
60. The diameter of the holes (11) can be in the range
1 to 30 mm, and the holes should be distributed evenly
over the conical surface to give an open area in the range
1 to 15% of the area of the surface.
~7 î~
~j v
It is not necessary for the diffuser to be in the
shap~ of a cone. Other shapes will perform as well,
providing the elevation of the underside of the diffuser
above the exit (5) of the vertical pipe (3), always
increases as the radial distance from the a~is of the pipe
increases. If the underside of the diffuser surface is at
any stage horizontal or tending to dip downwards as radial
distance from the vertical pipe increases, there will be a
tendency for the bubbles to collect in this area and
coalesce, and thereby form less surface area per unit of
gas volume, which will reduce the efficiency of any
contacting operation between the gas and the liquid.
An alternative form of the diffuser is shown in
Figures 2a and 2b, where the diffuser (12) takes the shape
of part of a circular dish, with holes (13) perforating
the dish. The radius of curvature of the dish can
conveniently be in the range 2 to 20 times the diameter of
the vertical pipe (3), and the diameter of the diffuser
(12) shown in Figure 2b should be in the range 1.5 to 20
times the outer diameter of the pipe (3), with good
practical results being found when the difuser diameter
is 2 to 3 times the pipe diameter. The diameter of the
holes (13) can be in the range l to 30 mm, and the holes
can conveniently be distributed evenly over the surface to
give a total area of the perforations in the range l to
20% of the area of the diffuser surface.
~ difficulty which can arise with the diffuser
arrangements as shown in Figures la and 2a is that solid
matter which may have been suspended in the liquid in the
reservoir ~4), may tend to settle on the upper face of the
diffuser (10) or (12) and build up into a thick layer
which may block the holes (11) or (13). This possibility
can be avoided by mounting the diffuser (10) or (12) so
that an annular gap (15) (Figures 3a and 3b) e~ists
between the diffuser and the pipe wall (3). The conical
diffuser is advantageous in this application because it
will provide a surface of constant angle to the
horizontal, down which any solids which may have deposited
on the upper surface may slide toward the a~is of the
cone. The half-angle of the cone, of which the diffuser
is a frustum, should be such that the solids will slide
toward the agis and hence fall through the annular gap
(15~.
It is advantageous to use a diverting ring (14) or
(16) in conjunction with the annular space ~15). The
purpose of the diverting ring is to prevent the dense foam
rising from the open end (5) of the pipe (3), from
entering the annular gap (15) and thereby evading the
diffuser (10) or (12). The ring is mounted on the outer
wall of the vertical pipe (3), and may conveniently be of
triangular section (14) as shown in Figure 3a or of
semi-circular section as in Figure 3b at (16).
Alternative embodiments are now described which
will improve the action of the liquid jet (B) shown in
Figure 1. When the system is in operation, the jet
plunges into the dense foam which fills the vertical pipe
(3), and gas which enters through the inlet ~6) is
entrained into the dense foam by the shearing action at
the edge of the jet. The velocity of the jet should be in
the range 3 to 40 metres/sec. If the ~elocity is too low,
the volume of air which can be entrained relative to the
volume of liquid supplied will be too little, whereas if
the velocity is too high, the energy demand will be
e~cessive. Good practical operational velocities are in
the range 12 to 20 metres/sec.
The jet diameter is fixed by practical considerations in
that if it is too small, there is the possibility of it
becoming blocked by adventitious material in the feed
liquid. The minimum diameter should be such that matter
suspended will pass through it. The diameter of the
vertical pipe (3) should be in the range 2 to 20 times the
jet diameter, with satisfactory operation being found in
the range 3 to 12 times the jet diameter.
Bubbles produced by the plunging jet are generated
by the shearing forces caused by the difference in
velocity between the jet and the dense foam into which it
. . -- .
12 -
` plunges. An important determinant of the ultimate size of
the bubbles is the power dissipated per unit volume of
fluid con-tained in the generating device. To define this
volume, use is made of the observation that the impinging
jet tends to spread out as it travels downwards in the
dense foam, giving up its forward momentum, and at a
certain point, the expanding jet comes into contact with
the wall of the vertical pipe (3~. The jet has been
observed to e~pand as a cone, whos2 included angle is in
the range 10 to 20. Thus for present purposes the
volume in which the energy contained in the jet is
essentially dissipated can be defined as the volume of
fluid contained in the vertical pipe t3), between the
entry point of the jet and the point at which the jet just
hegins to touch the wall of the vertical pipe (3).
The distance between the entry point of the jet and
the point at which the jet just begins to touch the wall
of the vertical pipe (3), is referred to as the
impingement distance, and it is desirable that the length
of the confining vertical pipe (3) should be greater than
the impingement distance. When it is, the initial
momentum of the jet is spread across the cross-section of
the pipe, and a two-phase mi~ture which is essentially
homogeneous has been created.
It will be evident that since the jet expands
slowly with distance from the orifice (2), the impingement
distance and hence the height of the vertical pipe (3)
could become excessive. One solution to this problem is
to use a multiplicity oE nozzles in a single vertical
pipe, and divide the flow between thern, as shown in Figure
4a, in which the liquid is fed through an entry pipe (1)
to a chamber (17), before issuing through a multiplicity
of nozzles or orifices. The number of jets is determined
by practical considerations, because the greater the
number of jets, the smaller will be the jet diameter for a
given flow rate, and the minimum jet size should be such
that it will not become blocked by solids suspended in the
liquid. The jet velocity is determined by the pressure in
- 13 -
the chamber (17), and is therefore the same for each jet.
A further improvement can be made by the
installation of vertical baffles in a multi-jet system, so
as to confine each jet within its own interior vertical
duct, as shown in Figure ~a and 4b. Without such baffles,
the individual jets are bounded mainly by the turbulent
fluid in neighbouring jets, and in part by the wall of the
confining pipe (3). The vertical baffles (203 provide a
solid physical boundary which fixes the region of energy
dissipation around each jet, and assists it to perform its
task of dividing up the entrained gas into fine bubbles.
The area of each section (21) of the cross-sectional area
of the pipe confined by the vertical baffles (20), should
be approximately the same.
Note that in Figure 4a, the nozzles (2) are shown
mounted on the lower extremity of short pipes (22), being
similar in construction to the single orifice case
depicted in Figure la. The purpose of the pipe piece (22)
is to allow the jets to commence at a level within the
pipe (3), which is below the air entry line (6). In
operation, the containing pipe (3) may fill with dense
foam up to the level of the entry point of the jet, and
hence liquid may flow back up the air line (6), in which
solids may be deposited. Accordingly, it is deisrable for
the individual liquid injection nozzles (2) to be below
the air inlet pipe (6).
The performance of the bubble dispersion system can
be enhanced in various ways depending on the interfacial
properties of the gas-liquid system. In order for a
stable two-phase mi~ture to fill the vertical pipe (33, it
is necessary that there should be little coalescence of
the bubbles as they are forced to flow downwards toward
the exit (53. Where coalescence occurs, very small
bubbles aggregate with others and grow into bubbles so
large that they may bridge the pipe (3), and cause
collapse of the two-phase mi~ture within the pipe.
Coalescence is prevented or inhibited by the
presence of salts, dissolved matter and especially
u~ - 14 -
surface-active agents dissolved in the liquid, as well as
the presence of particles of solid or insoluble liquids
such as oils and greases. Since the properties of each
gas-liquid system will be different, it is unlikely that
any single bubble dispersion device will be optimum for
all cases, and it may be necessary to modify the design to
cope with individual circumstances. A number of
modifications are now described which may be usefully
employed.
Figure 5 shows an arrangement in which the jet is
enclosed by a draft tube (30) in the form of an open-ended
cylinder, which extends down the axis of the vertical pipe
at least as far as the impingement point described above,
where the jet has expanded to occupy the full
cross-sectional area of the draft tube. The purpose of
the draft tube (30) is to restrict the volume of
gas-liquid mixture in the immediate vicinity of the jetj
so as to intensify the rate at which energy is dissipated
per unit volume of fluid, leading to smaller bubbles in
the vertical pipe (3). The diameter oE the draft tube can
conveniently be in the range 2 to 10 times the jet
diameter, with satisfactory operation being fcund in the
range 3 to 8 times the jet diameter.
The upper end of the draft tube may conveniently be
left open, or for ease of construction, it can be made in
the form of a cylindrical pipe attached to the head of the
pipe ~3) as shown in Figure 5, with a communicating
opening (31) being provided above the level of entry of
the liquid jet, in order to enable gas to recirculate
around the draft tube.
In a variation on this improvement, the draft tube
may be pierced with holes (32) which may occupy up to 20%
of the outer area of the tube as shown in Figure 5. The
purpose of these holes is to permit circulation of gas
which may return to the head space of the vertical pipe
(3) due to coalescence of the bubbles, while at the same
time providing sufficient integrity to the draft tube to
have a substantial confining action on the fluid within it.
~/f? r~ f~
- 15 -
Another variation which has been found useful is
depicted in Figure 6a, which shows a draft tube which is
mounted within the vertical pipe (3~. The top of the
draft tube is placed near the exit orifice (2) and the
sides of the upper part of the tuble (41) are slo~ing so
that its area increases to a point with increasing
distance down the vertical pipe (3). At a convenient
point (42), the area of the draft tube begins to contract
with distance down the pipe, and the lower part (43) of
:~0 the tube terminates at a convenient point so that the area
of the tube exit is larger than the cross sectional area
of the liquid jet at the same level. The purpose of this
form of draft tube, which first expands and then contracts
in area, is to permit any large bubbles or slugs of gas
which may have formed lower down in the vertical pipe (3),
to rise and bypass the jet, and then be re-entrained
through the entrance (45). Another form of this type of
draft tube is shown in Figure 6b, in which the draft tube
at its widest point essentially forms a seal against the
inner wall of the vertical pipe (3). Thus any large
bubbles which have risen up the walls of the pipe (3) are
trapped in the annular space (44) between the draft tube
and the pipe, and are re-entrained through the ring of
openings (45~ located immediately beneath the widest part
of the draft tub~ (44).
The alternative configuration shown in Figure 7 is
intended where minor coalescence takes place, and the
bubbles are r/ot so large as to cause collapse of the
two-phase medium within the vertical pipe (3), but which
may benefit from fu~ther exposure to the high-speed jet.
In this alternative, the draft tube first contracts in
area as distance increases down the pipe (3~, to reach a
minimum at the point (52), below which it increases in
area. The cross-sectional area of the open tube at the
point ~5~ should be greater than the cross-sectional area
of the e~panding liquid jet at this point. Annular spaces
(53) and (54) may be left between the draft tube and the
inner wall of the pipe (3), or the draft tube may be
... .
- 16 -
sealed to the walls of the pipe (3).
The dimensions of the vertical pipe (3) relative to
the diameter of the jet, have an important bearing on the
operation of the bubble dispersing system. For
satisfactory operation, it has been found that the
diameter of the vertical pipe (3) should be in the range 2
to 20 times the jet diameter, with satisfactory operation
being found in the range 3 to 12 times the jet diameter.
In many cases, the operating gas will be air at
atmospheric pressure, and it is advantageous if the device
can operate by drawing air from the atmosphere without the
need for a blower or compressor. This can be achieved if
the pressure in the vicinity of the jet orifice (2) is
less than the atmospheric pressure.
In other situations, however, air may be supplied
into the upper part of the conduit or pipe (3) under
pressure via a blower or compressor. This arrangement may
be applicable where it is desired to submerge the pipe (3)
to a larger degree within the body of liquid (4), so
requiring a greater "head" within the top of the pipe (3)
to cause the foam to move downwardly within the pipe and
issue from the lower end ~5) against the pressure head in
the body of liquid (4).
Although the invention has been described in one
preferred form using air as the gas for "aeration" it will
be appreciated that other gases may be used in certain
situations where it is required to "aerate" a liquid with
a gas other than air.
Although the invention has been described with
reference to the aeration of wastewaters, it is also
suitable for the flotation of mineral particles so as to
remove the valuable minerals from unwanted waste matter,
by contacting them with fine bubbles in a suspension of
the mineral in water, so that the particles which it is
desired to remove have been rendered non-wetting by the
liquid while the particles which are to remain in the
liquid are rendered wettable by the liquid. The valuable
particles then adhere to the surface of the fine bubbles
.
.
- 17 -
and rise with them to the surface of the liquid, from
which they may be removed as a froth.
